The present invention relates to computer recognition systems, and more particularly to systems that record the passing of a vehicle at one camera location and then recognize the same vehicle passing a second camera at another location.
A few commercial systems have become available for detecting traffic violations and imaging cars involved in the violation. A video image or photograph that clearly shows the vehicle involved and its license plate is needed to sustain a conviction in court. This is more difficult to accomplish than may be thought because lighting conditions can vary greatly and cameras usually have a very limited dynamic range. The contrasting color choices made by the states and other governments add to the problem, and so white light is necessary to gain a high contrast picture under all color combinations.
For example, the Pulnix America, Inc. (Sunnyvale, Calif.) markets a vehicle imaging system (VIS) that is a low-cost imaging solution for violation enforcement systems, speeding, red light, bus lane, railroad crossing, automatic license plate readers, electronic toll collection, and lane-based open road video billing. Such VIS must produce legible images of passing vehicles and their license plates under all weather and lighting conditions. Retro-reflective and non-retro-reflective plates, including older plates with diminished retro-reflective properties, are imaged by the VIS day or night. Unlike most other imaging systems, the VIS is said to adapt instantly to changing lighting conditions and ignore road reflections. It focuses instead on what is important, the brightness of the car and its plate by using a smart light sensor. Such measures the instantaneous dynamic range of plate/vehicle brightness and electronically adapts camera parameters, e.g., gain and shutter speed, to achieve optimal imaging of the vehicle and its license plate.
In order to achieve the highest levels of accuracy in detection, these cameras must have high speed electronic shutters, high resolution, high frame rate, and communication capabilities. A progressive scan interline transfer CCD-type camera, with its high speed electronic shutter and resolution capabilities, provides the basic functions to meet the requirements of a traffic camera system. Traffic cameras must deal with harsh environmental conditions and an extremely wide range of light.
Optical character recognition (OCR) is a critical function of a modern traffic camera system, with detection and accuracy heavily dependent on the camera function. In order to operate under demanding conditions, communication and functional optimization is implemented to control cameras from a roadside computer. The camera operates with a shutter speed faster than {fraction (1/2000)} second to capture highway traffic both day and night. Consequently camera gain, pedestal level, shutter speed and gamma functions are conventionally controlled by a look-up table containing various parameters based on environmental conditions, particularly lighting. Lighting conditions are studied carefully, to focus only on reading the critical license plate surface.
It is therefore an object of the present invention to provide a vehicle video imaging system that can produce clear pictures of a vehicle and its license plate.
It is a further object of the present invention to provide a vehicle video imaging system that automatically adjusts for retro-reflective and non-retro-reflective license plates on a vehicle being imaged.
Briefly, a vehicle video imaging system embodiment of the present invention comprises a white-light LED array for illuminating retro-reflective painted parts of a vehicle""s license plate, a powerful flash with a visual spectrum cutout filter and a polarizing filter for illuminating any non-retro-reflective license plate paint and the vehicle itself. A video camera with a polarizing filter turned 90xc2x0 relative to the one in front of the flash receives the illuminated image of the vehicle and its license plate. The retro-reflective paint of a license plate will return polarized light as it is received, so the white-light LED array will provide all the illumination needed by the camera to get a good high-contrast picture of the license plate. The polarizing filters will combine to block out most of the light from the flash that was returned still-polarized by the retro-reflective-paint license plate. All other surfaces that do not have retro-reflective paint will bounce-back and scatter the light from both the polarized flash and non-polarized light.
An advantage of the present invention is that an imaging system is provided that produces good contrast pictures of vehicles and their license plates.